All table widths are set here to 100%, but its easy to change if you delete line 772 (.entry-content table,)
or write your own arguments between .entry-content table and .comment-content table inside a new {}

Do not edit the Twenty Eleven theme. It is the default WordPress theme and having access to an unedited version of the theme is vital when dealing with a range of site issues. Create a child theme for your changes. Once your child theme is active, we should be able to help with the customisation.